Pre-university Advanced Level Organic Chemistry: The uses of aldehydes and ketones
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Part 5. The chemistry of ALDEHYDES and KETONES - USE Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry
All revision
notes on the chemistry of aldehydes and ketones
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mobile phone or ipad etc. in 'landscape' mode This is a BIG website, you need to take time to explore it [SEARCH BOX] Part 5.8 The uses of aldehydes and ketones and their occurrence in nature - natural products of biochemistry Use as solvents themselves Aldehydes are not used as solvents - unpleasant and can be harmful if inhaled. The lower ketones are like propanone and butanone are very useful solvents because they dissolve a wide variety of organic compounds, both polar and non-polar organic molecules.
Flavourings and perfumes The aromatic aldehyde Many more complex molecules
containing an aldehyde group or ketone group occur in essential oils from plants, often
giving, or contributing to, their characteristic odour e.g. citral and
carvone whose skeletal formulae are shown below.
There are many aldehydes and ketones in the natural products
of plants including terpenoids - a very diverse group of compounds
contributing to the smell and taste of many natural products and perfume
formulations. These are unsaturated compounds (C=C functional
group), but also contain an aldehyde of ketone group. Some of
these terpenoids are illustrated below.
A Citral
(lemonal), C10H16O,
is an aldehyde with all the usual characteristic reactions of an R-CHO
molecule. Citral also has two alkene groups (2 x C=C, diene) as well as the
aldehyde group.
It is found in several species of lemon plants (e.g. lemon grass oil) and contributes to the
strong citrus 'lemon-like' odour of the fruit.
Citral can exhibit E/Z stereoisomerism via the top C=C bond (geometric
isomers) because this alkene group is not part of a ring system..
Citral cannot exhibit R/S stereoisomerism - no chiral carbon.
Note 'al' in the name 'citral' indicating an aldehyde.
B Carvone, C10H14O, is a cyclic ketone found in
spearmint and caraway oil. It has one ketone group and behaves as a ketone, but it
also two C=C double bond groups, so is also a 'diene'. Both
citral and carvone behaves as a 'double' alkene, reacting quantitatively
with two molecules of bromine.
Carvone cannot exhibit E/Z stereoisomerism via the top C=C bond
(geometric isomers) - one C=C has two identical end groups (H) and the top
left C=C is part of the ring. Citral can exhibit R/S stereoisomerism - it has a chiral carbon - the
bottom one of the hexagonal ring - see diagram on right for the
non-superimposable mirror image forms. Note 'one' in the name 'carvone'
indicating a ketone. 'Vanilla' one of our favourite
tastes!
Natural vanillin is extracted from the seed pods of
Vanilla planifolia, a vining orchid native to Mexico, but now grown in
tropical areas around the globe. The harvested green seed pods contain
vanillin in the form of a glucoside, but the green pods do not have the
flavour or odour of vanilla. Vanillin glucoside is shown on the top left
of the diagram
1. Vanillin glucoside
consists of the basic vanillin molecular structure joined to cyclic
hexose sugar molecule).
2. From the natural sourced vanillin glucoside, vanillin (methyl
vanillin, 4-hydroxy-3-methoxybenzaldehyde) is extracted.
3. The production of vanillin from natural sources is
quite expensive, so several cheaper synthetic routes have been devised
including using 2-methoxyphenol which is another synthetic product of
the petrochemical industry.
4. 'ethyl vanillin',
3-ethoxy-4-hydroxybenzaldehyde, is a synthetic analogue of vanillin with
a much stronger vanilla flavour and odour than natural vanillin. Using
synthetic 'vanilla' molecules is cheaper, but those with sensitive taste
buds might be able to tell the difference from naturally sourced vanilla
flavouring. For 4. note the
change in order of the substituents from 2. due to the IUPAC alphabetical
nomenclature rule. A nose footnote!
Much of our sensation of taste is actually due to the smell of the
food we are eating. When eating, we smell these
flavours (aldehydes, ketones and
esters too - think of
fruity odors/tastes etc.) and these odours make the food appetising.
The flavour molecules like vanillin, are sufficiently volatile to
reach the olfactory receptors in the nose - note that if you have a
heavy cold and blocked nose, your appetite seems decreased and the
food doesn't taste as good.
Other naturally occurring aldehydes and
ketones
Cinnamaldehyde, molecular
structure C6H5-CH=CH-CHO (check versus skeletal
formula),
is an aldehyde found in the oil of cinnamon bark and has been traditionally
used in medicines to treat coughs and sore throats. Cinnamaldehyde can exhibit E/Z
isomerism, the natural product is the E isomer (trans, shown
above). It is used as a flavouring in
sweets and beverages and a fragrance additive in perfumes. It has also been used as a safe
insecticide against mosquitos and also has antibacterial and
antifungal properties. Octanal, molecular structure CH3(CH2)6CHO,
is found in orange juice and contributes to the fragrance of oranges and
also to roses and violets.
Polymers
The plastic Bakelite is made by a reaction between
methanal (formaldehyde) and phenol.
Preservatives,
herbicides - many aldehydes are harmful and some toxic Methanal
(formaldehyde) solution is used to
preserve anatomical specimen - never forgot the smell of preserving the body
of a rat I dissected as a student !!! It can also be used to sterilise
surgical equipment - antiseptic/disinfectant properties. Propenal ('Acrolein') is an unsaturated
aldehyde (H2C=CH-CHO) is used as a contact herbicide |
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or
benzaldehyde, C7H6O,
has the smell of almonds and is synthesised for use in flavouring food - I love thick
layer of marzipan between a thin slice of sugar icing and a thick layer
of rich Christmas cake!
Phenylethanone (left, acetophenone) is a ketone used in fragrances to smell like cherry, honeysuckle, jasmine and strawberry.
They are also both a good
test in reading skeletal formulae - match them up with the molecular
formulae!
